glsl/linker: always validate explicit locations for first and last interfaces

glsl/linker: always validate explicit locations for first and last interfaces

Until now, we were only doing this when linking a SSO
program. However, nothing avoids linking a non SSO program which
doesn't have both a VS and FS. In those cases, we also need to report
the usual linking errors, if happening.

v2: Use a better name for the renamed function (Timothy).

Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>